name can also be seen as a reference to "walking through the head" ("Headwalker"(Dutch: "Doorloopkop")). When ICM units joined, the doors at the end of the train would open, and a walkway would emerge, coupling with the other set and allowing passengers and crew to walk through to the next set. (This makes it easy for conductors to check tickets and check on the security of the passengers. In addition, each set had catering cars before they were abolished in 2003). Starting from 2005 these gangways were no longer used. The reason given was that they were not often used, but they also had many technical problems that made the trains late on a regular basis. During their refurbishment from 2006 onwards, these hallways were permanently sealed shut and the doors at the heads of the train were replaced by a light polyester plate, saving the NS maintenance costs. Refurbishment and renewal From November 2006, the Koploper went through a process of refurbishment. The first train left NedTrain's workshop in Haarlem in April 2007 and then put into the regular service. To distinguish the not yet modernized sets from the modernized sets, modernized train sets are referred to as ICMm. These sets feature new interiors with 13% more seats. One could distinguish apparent differences between 1st and 2nd class. The first class consists of red seats and second class consists of blue seats, equal to SGMm units. Other improvement include the addition of a wheelchair toilet, air conditioning, OBIS, and digital travel information. On the exterior, the train sets were repainted showing blue window surrounds curving at the ends of units. Numbers are now nearer the front of the train. The connectors diverting the train compartments were revealed and digital destination screens were put at the trains ends. The renewal of the series 4011–4097 was completed in June 2010. Refurbishing and renewing of the 4-car units took place between February 2010 and July 2011. Adverts Between 1986 and 2002 many sets featured adverts. In 1986, in relation to the opening of the Amsterdam–Schiphol railway line, two units were painted in the colours of airplanes; 4011 was painted in KLM colours and 4012 in the colours of Martinair 4011, 4012 and 4024 were painted in the colours of Aegon 4050 was painted in the colours of Randstad and later in De Lage Landen colours 4023 advertised the Groninger Museum, but in